1. Field of the Invention
This invention relates to a microsystem with a flexible membrane for a pressure sensor and its manufacturing process.
A microsystem with a flexible membrane refers to a pressure sensor cell, for example of a capacitive measurement type, or a switch cell triggered by pressure.
The invention has applications in the manufacture of microsensors, microswitches, variable microcapacitors, and more generally micro-components that can be integrated with an associated electronic circuit. One particular application of the invention is the manufacture of a fingerprints sensor.
2. Background Art
French patent publication 2 700 003 published Jul. 1, 1994, describes the manufacture of pressure micro-sensors with a deformable silicon membrane.
Due to their manufacturing process, compatibility difficulties occur when making this type of microsensor jointly with CMOS type integrated circuits. Furthermore, these sensors include a generally conducting membrane, which may cause electrical insulation problems when components are miniaturized.
Insulating membrane sensors made on glass substrates are also known. For example, this type of sensor is described in Canadian patent publication CA 2,130,505, published Aug. 19, 1994.
The reference entitled xe2x80x9cA Surface Micromachined Miniature Switch for Telecommunication Applications with Signal Frequencies from DC up to 4 GHzxe2x80x9d from J. Jason and F. Chang, International Conference on Solid State, Sensors and Actuators, Stockholm, Sweden, Jun. 25-29, 1995, describes the manufacture of sensors associated with electronic circuits and adjacent to each other on the same substrate. This type of design is particularly attractive for production manufacturing.
However, it has a number of limitations concerning interconnection problems between sensors and the corresponding electronic circuits. The electrical connections between the sensors and integrated circuits placed on the same substrate generate harmful parasite capacitance effects. Furthermore, the electrical connections are cumbersome and form an obstacle to increased miniaturization of the devices.
Finally, separate manufacturing of sensors and associated electronic circuits result in high manufacturing costs.
The purpose of this invention is to propose a process for manufacturing Microsystems with a flexible membrane that do not have the limitations mentioned above. In particular, one purpose is to propose microsystems with a flexible membrane made integrally on a substrate according to a process compatible with the manufacture of MOS integrated circuits in the same substrate.
Another purpose is to propose pressure sensors or integrated microswitches that can be laid out in the form of a large number of adjacent cells, for example in the form of a matrix.
Another purpose is to propose a fingerprints detector using this type of sensor.
Finally, another purpose is to reduce the manufacturing cost of Microsystems with flexible membranes.
In order to achieve these objectives, the purpose of the invention is more precisely a process for manufacturing a microsystem for a pressure sensor comprising the following steps:
a) deposit and forming of at least a first conducting layer on a support, the conducting layer forming at least a first electrode,
b) deposit and forming of at least one layer of sacrificial material covering the first conducting layer,
c) deposit and forming of a second conducting layer on the layer of sacrificial material in a region located above the first conducting layer, the second conducting layer forming a second electrode,
d) formation of a first membrane layer covering and surrounding the layer of sacrificial material and the second conducting layer,
e) elimination of the layer of sacrificial material,
f) forming the first membrane layer.
The steps in the process are preferably carried out in the order mentioned above. However the order of steps e) and f) can be reversed. Furthermore, in one special embodiment, the layers mentioned in steps b) and c) may be formed at the same time.
In particular, when major cutting or thinning operations are necessary on the first membrane layer, it is useful to not release it by eliminating the sacrificial layer, until it has been completely formed.
The layer of sacrificial material is formed so as to define the shape and size of a closed chamber, in which one wall is formed by the first membrane layer.
The process according to the invention can be used to make simple and inexpensive Microsystems, and particularly cells for pressure sensors suitable for common integration with CMOS or BICMOS type circuits on the same substrate. This aspect will be described in more detail in the rest of the description.
Beneficially, with the process according to the invention, the first and second conducting layers forming the electrodes are not separated by any material layer. After the layer of sacrificial material has been eliminated, the second conducting layer is kept separated from the first conducting layer by means of the flexible membrane layer, when there is no pressure exerted on this membrane layer.
A particularly sensitive sensor cell can be made by adjusting the thickness of the layer of sacrificial material.
Furthermore, the first and second layers of conducting material may be separated by a very small distance only, for example of the order of about 0.1 xcexcm to 5 xcexcm. This characteristic is also useful for making sensitive sensors. For example, the first and second layers could form the armatures of a capacitor in which the capacitance varies as a function of a deformation of the membrane layer.
In other applications, the microsystem obtained by the process according to the invention could also form a microswtich. The first and second conducting layers may in this case form the terminals of the switch. For example, this type of switch is open when the deformation of the membrane layer is insufficient for the first and second conducting layers to come into contact with each other, and is closed when the conducting layers are pressed in contact with each other.
According to one alternative embodiment of the microsystem for use as a switch, a groove is formed in the first layer of conducting material to separate and delimit two electrodes, and the second conducting layer is then formed on the layer of sacrificial material in a region above at least one portion of the groove.
In a microsystem made according to this alternative, when the membrane layer is deformed, the second conducting layer electrically connects the two electrodes in the first conducting layer which thus form the terminals of the switch.
According to one specific embodiment of the process according to the invention, elimination of the layer of sacrificial material may include opening of at least one etching channel through the first membrane layer, etching of the layer of sacrificial material and the formation of a second membrane layer covering the first membrane layer and closing the etching channel, the second membrane layer also being formed during step f) in the process.
For example, step f) may include partial elimination of the second membrane layer and thinning of the first membrane layer in a region surrounding the second electrode.
Thinning of the first membrane can precisely adjust its suppleness or flexibility and therefore the sensitivity of the microsystem.
Preferably, thinning takes place in a region surrounding the electrode formed in the second conducting layer but not above this electrode. This characteristic increases the stiffness of the membrane in the region of the second electrode and thus prevents excessive deformation of this electrode when pressure is applied to the membrane.
Preferably, formation of the first membrane layer may include the successive deposit of three sublayers of material, in which at least one sublayer forms an etching stop sublayer. The first membrane layer is then thinned by etching, stopping the etching on the etching stop sublayer.
Thus the final thickness of the membrane layer may be very small without risking penetration during etching. Furthermore, the thickness may be determined very accurately.
According to another advantageous aspect, the channel for eliminating the layer of sacrificial material can be formed in a region in the first membrane layer located above a peripheral part of the layer of sacrificial material.
This arrangement avoids the need to increase the stiffness of the membrane if the material in the second membrane layer enters the channel and forms a plug supported on the support of the first conducting layer. Formation of a plug in a peripheral region of the membrane only very slightly affects the flexibility of the membrane.
According to one particular embodiment of the process according to the invention, forming of the first membrane layer may also take place before the layer of sacrificial material is eliminated. In this case, forming of the membrane layer may include etching of one or several channels in the form of through trenches reaching the layer of sacrificial material.
These trenches are used to eliminate the layer of sacrificial material.
They can also be used to partly release part of the membrane layer, when the sacrificial material has been eliminated. The trenches may be laid out to define part of the perimeter of a portion of the membrane layer. The trenches are then preferably above a peripheral part of the layer of sacrificial material.
The first conducting layer may be made flush with the support surface. In this case, in step a) the first conducting layer is formed in a recess formed in the support such that the first conducting layer and the support have flush surfaces.
This type of construction may be beneficial when the microsystem is used as a sensor cell with capacitive measurement of the membrane deformation.
However, when the microsystem is used as a switch, it is preferable that at least the first or the second electrode projects above its support.
This can be done, for example by forming the second conducting layer in a recess formed in the layer of sacrificial material, in a region located above the first conducting layer.
According to one possible alternative, step b) includes the formation of a first layer of sacrificial material surrounding at least part of the first conducting layer and then a second layer of sacrificial material is formed covering the first layer of sacrificial material and the said part of the first conducting layer, and the first and second layers of sacrificial material are formed. Due to this arrangement, the second layer of sacrificial material has a recess on its free upper surface that is filled by the second conducting layer. Thus after the sacrificial material has been eliminated, the second conducting layer projects on the surface of the membrane layer facing towards the support.
The invention also relates to a pressure sensor cell comprising:
a substrate comprising at least a first electrode,
a deformable membrane fixed by an edge around the periphery of the substrate and defining a closed chamber around at least part of the first electrode,
a second electrode formed on a wall of the deformable membrane facing towards the first electrode and kept separate from the first electrode in the lack of any pressure exerted on the membrane.
The pressure sensor cell may also comprise or be associated with an electronic measurement circuit integrated into the substrate and connected to the first and second electrodes.
Beneficially, the electronic circuit may be placed in the substrate under the first electrode.
A better integration of the cell and the integrated circuit may be obtained due to this characteristic.
In particular, this means that a fingerprints sensor comprising a large number of adjacent sensor cells can be made, for example in the form of a matrix. All cells and the associated electronic circuits can then be made on or in the same support.
Note that Microsystems and cells according to the invention are particularly suitable for use with CMOS (Complementary Metal Oxide Semiconductor) or BiCMOS (BipolarCMOS) type integrated circuits.